The present invention relates to X-ray imaging systems, and particularly to devices for evaluating the operation of the imaging system.
Medical imaging systems, such as used in mammography, transmit X-rays from a source through the object being imaged and on to photographic film in a cassette. As much as 95 percent of the X-rays pass through the photographic film without producing a significant change in the film. In order to enhance the image on the photographic film, an intensifying screen is placed in the cassette in contact with the film. This screen comprises a transparent sheet on which is deposited a layer of phosphor that produces visible light when bombarded with X-rays and which is much more responsive to X-rays than the photographic film. The film reacts to the light from the screen as in conventional photography. Thus, when the photographic film is developed, the areas exposed to greater amounts of light will be darker than those areas which were exposed to less light.
In the United States, mammography X-ray systems have to be certified periodically by the U.S. Food and Drug Administration. Such certification involves producing a test image of a "phantom" object which simulates human tissue using a defined set of parameters for exciting the X-ray tube. The radiation exposure is often measured by a thermal luminescent dosimeter (TLD) that contains lithium fluoride crystals which, when heated after being exposed to X-rays, emit light in an amount that corresponds to the magnitude of the X-rays that struck the dosimeter. The certification process involves sending the exposed dosimeter to a designated laboratory which determines the X-ray exposure by measuring the amount of light given off by the crystals upon heating. The photographic film which recorded an image of the test phantom is sent to an accrediting agency recognized by the U.S. Food and Drug Administration which also receives the X-ray exposure measurement from the laboratory. This certification method requires separate processing of the photographic film and the thermal luminescent dosimeter and uses separate paths to deliver the results to the accrediting agency.
At other times, it is desirable for quality control purposes that the X-ray equipment user be able to conduct test exposures to evaluate the performance of the X-ray imaging system and the film processor that develops the photographic film. For such quality control tests, photographic film may be exposed using a phantom that contains an attenuating step wedge to produce an image on the film having areas of different optical densities. It is desirable to know the X-rays exposure of the test procedure in order to correlate the densities of the developed film to an amount of radiation to which the screen-film combination was exposed.